For carbon nanotubes (CNT) for which development of functional materials has been expected as new electronic device materials, optical device materials, conductive materials, bio-related materials, etc., studies have been progressed earnestly for yield, quality, use, productivity, production process, etc.
One of methods for producing carbon nanotubes includes a chemical vapor deposition (CVD) method (hereinafter also referred to as a CVD method) and the method has attracted attention as being suitable to mass synthesis. The CVD method has a feature of bringing a carbon compound as a carbon source into contact with fine metal particles as a catalyst at a high temperature of about 500° C. to 1,000° C., and various variations are possible depending on the kind and the arrangement of the catalyst, kinds of carbon compounds, and conditions and production of single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) is possible. Further, it has an advantage capable of growing by disposing the catalyst on a substrate.
However, in the production of the carbon nanotube according to the existent CVD method, since the catalyst or by-products intrude into the formed carbon nanotube, purifying treatment of applying various chemical treatments have been necessary in order to obtain a highly pure carbon nanotube from the product. The purification treatment includes a plurality of complicate and expensive processes, for example, an acid treatment in combination, and required considerable skills and increase in the cost of obtained products. Further, even when such purification treatment is applied, the purity is restricted to about 90 to 94 mass % and it was difficult to obtain a single-walled carbon nanotube at high purity of 98 mass/% or more (Nanoletters 2, 385 (2002)). Further, chemical and physical properties of the carbon nanotube often changed in purification making it difficult to easily obtain carbon nanotubes always at a determined quality.
Further, the growth of the carbon nanotubes by the existent CVD method involved a problem that the activity lifetime of the metal catalyst was short, the activity was degraded in several seconds to several tens seconds and the growth rate of the carbon nanotube was not so great, which hindered the productivity.
With the situations described above, it has been proposed a method of controlling the activity of the iron catalyst and the growth of the carbon nanotube by preparing a catalyst by dipping a substrate in an aqueous mixed solution of FeCl3 and hydroxylamine (Hee Cheul Choi, et al., NANO LETTERS, Vol. 3, No. 2, 157-161 (2003)).
However, with all such a proposal, extension of the lifetime of the catalyst activity and increase in the growth rate have not yet been satisfactory also with a practical point of view at present.
On the other hand, among the carbon nanotubes, single-walled carbon nanotubes have attracted attention as a material for nano-electronic devices or nano-optical devices and energy storage in view of extremely excellent electrical property (extremely high maximum current density), thermal property (heat conductivity comparable with diamond), optical property (light emission at an optical communication band wavelength), hydrogen storing performance, and metal catalyst supporting performance. In a case of effectively utilizing the single-walled carbon nanotube as the material for the nano-electronic devices, nano-optical devices, energy storage, etc., it is desired that aligned single-walled carbon nanotubes form a bulk structure in the form of an aggregate comprising aligned single-walled carbon nanotubes gathered in plurality, and the bulk structure provides electric, electronic, optical, and like other functionality. Further, such carbon nanotube bulk structures are desirably aligned in a predetermined direction, for example, as in vertical alignment, and the length (height) is desirably large scaled. As bulk structures in which a plurality of vertically aligned single-walled carbon nanotubes are gathered reported so far, those with a height reaching 4 μm have been reported (Chem. Phys. Lett. vol. 385, p. 298 (2004)).
By the way, for applying the vertically aligned single-walled carbon nanotubes for nano-electronic devices, nano-optical devices or energy storage in practical use, a further large scale is necessary.
Further, those having a plurality of vertically aligned carbon nanotubes formed into the bulk structure and patterned are extremely suitable to the application use for the nano-electronic devices, nano-optical devices, or energy storage as described above. However, while patterning has been attained in some multi-walled carbon nanotubes (Science 283, 412 (1999)), attainment of patterning for the single-walled carbon nanotubes has not yet been reported.
Compared with the multi-walled carbon nanotube, since the single-walled carbon nanotube is excellent in properties such as more excellent strength property, bending property, flexing property, higher optical transmittance, larger specific surface area, more excellent electron emitting property, more excellent electric conductivity and, further, presence of both semiconductors and metals while the multi-walled carbon nanotube is formed of a metal, it has been expected that when such vertically aligned single-walled carbon nanotube bulk structure is formed, application use to nano-electronic device, nano-optical device, energy storage, etc. will be increased outstandingly. However, the single-walled carbon nanotubes tend to be adhered intensely to each other because of a strong Van der Waals force and constitute a disordered and non-aligned bulk structure, for example, during purification for removing impurities and re-construction of once formed not-order and not-alignment bulk structure is extremely difficult, for example, due to the difficulty in dispersing the single-walled carbon nanotubes in a solution and, with the reasons, such a bulk structure has not yet been obtained at present.